1. Field of the Invention
This invention relates to the field of flow control in a fluid. More specifically, the invention comprises the use of a multi-stage microjet-based actuator to create a highly unsteady flow field.
2. Description of the Related Art
Active control of fluid flow has many applications. One particular application involves noise suppression for aircraft. Another application is the control of flow separation over airfoils and lifting bodies. Because such flows typically involve rapid fluctuations, an actuator intended to achieve active control must be very responsive. Such an actuator must be able to create rapidly changing (highly unsteady) fluctuations in the flow.
Large scale supersonic impinging jets are known to create a highly unsteady flow field with a high mean and unsteady momentum. This flow field contains periodic pressure variations centered on certain frequencies. The same is true for small scale impinging jets. A supersonic microjet having a nozzle pressure ratio of 5.8 impinging upon a plate produces strong impinging tones in the range of 25-55 kHz. The resonance loop seen in larger jets is therefore also present in microjets.
The inventors have previously studied the effects of a microjet directed through a hole in a plate. Such a flow produces edge/hole tones. If the microjet's shear layer grazes the edge of the hole large amplitude tones—referred to as “hole tones” are produced. The hole tones tend to be lower in amplitude than simple impingement.
Still others have investigated the effects of a microjet directed into a cylindrical cavity having a closed bottom (a “blind hole”). This flow produced high amplitude tones in a suitable range of frequencies (“suitable” in terms of their possible application to active flow control). These prior results led the inventors to create the present invention, which serves the need for an actuator which can produce high-amplitude disturbances over a wide range of frequencies.